Communication terminal

ABSTRACT

A communication terminal is capable of simultaneously communicating through a plurality of carriers by carrier aggregation, and the communication terminal comprises: a quality measurement unit for measuring the reception quality of a radio wave transmitted through a plurality of carriers from a base station of a connected cell to obtain a measured value; a primary carrier storage unit storing information specifying a primary carrier chosen from the plurality of carriers; a comparator for comparing a measured value of the primary carrier measured by the quality measurement unit to a threshold value; and a cell search unit for searching for another cell when the measured value of the primary carrier is less than or equal to the threshold value. Consequently, a search threshold value for carrier aggregation can be appropriately determined to perform a cell search and a quality measurement.

RELATED APPLICATIONS

This application claims the benefits of Japanese Patent Application No.2009-147778 filed on Jun. 22, 2009 in Japan, the contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a communication terminal and a basestation for controlling a communication terminal that support componentcarrier aggregation (also called just “carrier aggregation,” “bandaggregation,” or “band bonding”).

BACKGROUND ART

In a mobile communication system, a communication terminal searches fora neighboring cell and measures the reception quality of a radio wavefrom the detected neighboring cell (hereinafter referred to as “qualitymeasurement”) when there is a decline in the quality of communicationwith a cell it is currently connected to (hereinafter referred to as a“serving cell”). If a neighboring cell of better reception quality thanthe serving cell is found as a result, a network controller causes thecommunication terminal to perform a handover to the neighboring cell.

The search and quality measurement for a neighboring cell are majorfactors in terms of reducing the power consumption of a communicationterminal. Basically, if a serving cell is of a sufficiently goodquality, the communication terminal is presumed to have no need forperforming the search and quality measurement for a neighboring cell,since a communication terminal only needs to be connected to a servingcell. A threshold value for determining whether or not to perform aneighboring cell search (this threshold value is called “S-measure” inLTE) is therefore specified (Non-patent document 3). This thresholdvalue is herein called a “search threshold value.”

FIG. 24 illustrates a search threshold value. As shown in FIG. 24, whenthe measured value of the reception quality of a serving cell is above asearch threshold value, a neighboring cell search is not performed sincethe quality is good and a handover is presumed to be unnecessary. On theother hand, when the measured value of the reception quality of aserving cell is below a search threshold value, a neighboring cellsearch is performed since the quality is bad and a handover may be made.Consequently, a neighboring cell search is performed only when required,and the power consumption of a communication terminal can be reduced.

By the way, LTE-advanced is now being standardized by 3GPP so as to be acandidate for a wireless communication system adopted into IMT-advanced.In this LTE-advanced standardization, carrier aggregation, in which aplurality of component carriers are simultaneously assigned to acommunication terminal, is under review for improvement in thethroughput of a communication terminal.

FIG. 25 is a conceptual diagram illustrating carrier aggregation. In theexample shown in FIG. 25, there are component carriers f1 to f3 of abandwidth of 20 MHz. A communication terminal supporting carrieraggregation (e.g. a Rel-10 communication terminal) uses the componentcarriers f1 to f3 simultaneously to communicate with a bandwidth of 60MHz.

On the other hand, a communication terminal which dose not supportcarrier aggregation (e.g. a Rel-8/9 communication terminal) connects toone of the component carriers f1 to f3 to communicate over 20 MHz.

Keeping the bandwidth unchanged as above allows previously releasedcommunication terminals (e.g. Rel-8/9) to be supported as well, and canimprove the throughput of communication terminals to be newly released(e.g. Rel-10 communication terminals). This is one merit of carrieraggregation.

Note here that a communication terminal incompatible with carrieraggregation regards each circle of the carriers f1 to f3 shown in FIG.25 as a cell. The cell is defined by 3GPP (Non-patent document 1).Further efficiency is now under study in consideration of implementingcarrier aggregation. Scenarios for enhancing efficiency will bedescribed below.

(Scenario 1)

FIG. 26 shows one scenario for further enhancing the efficiency ofcarrier aggregation. A component carrier f1 includes a synchronizationchannel, broadcast information, an L1 control channel, and the like, andcan alone provide services to a communication terminal. Componentcarriers f2 and f3 do not include a synchronization channel norbroadcast information, and a communication terminal cannot detect thosecomponent carriers alone. This is because a communication terminaldetects a component carrier (which is called “cell detection” in Rel-8)by receiving a synchronization channel in a cell search process.

A communication terminal cannot be on standby (which is called “campon”) nor establish a call on the component carriers f2 and f3. Thestandby and call establishment are allowed by receiving broadcastinformation (more specifically, Master Information Block (MIB), SystemInformation Block 1 (SIB1), and System Information Block 2 (SIB2) inbroadcast information) after cell detection. A communication terminaltherefore cannot be on standby on the component carrier concerned unlessthere are both a synchronization channel and broadcast information.

In this scenario, a communication terminal in an idle state (RRC_IDLE)detects only the component carrier f1 and then begins to be on standby.After that, the communication terminal performs a call establishmentprocess, comes into an active state (RRC_CONNECTED), and then adds thecomponent carriers f2 and f3 in accordance with an instruction from thenetwork side, to perform carrier aggregation. Since the communicationterminal may require reception of broadcast information even after itcomes into an active state, there may be an operation in which thecommunication terminal continues to use the component carrier f1 anduses the component carriers f2 and f3 just as additions. FIG. 27 showsone example of a process of adding the component carriers f2 and f3.

A communication terminal which does not support carrier aggregation(e.g. a Rel-8/9 communication terminal) will use only the componentcarrier f1 even after it comes into an active state.

(Scenario 2)

FIG. 28 shows another scenario for further enhancing the efficiency ofcarrier aggregation. A component carrier f1 includes a synchronizationchannel, broadcast information, an L1 control channel, and the like, andcan alone provide services to a communication terminal. Componentcarriers f2 and f3 do not include an L1 control channel, and acommunication terminal cannot detect those component carriers alone.This is because a communication terminal cannot determine which resourceit should use when there is no L1 control channel, since it is notifiedof which resource it should use through an L1 control channel.

As with the previously described scenario, a communication terminal inan idle state cannot be on standby on the component carriers f2 and f3,and a communication terminal which does not support carrier aggregation(a Rel-8/9 communication terminal) also cannot use the componentcarriers f2 and f3.

In the above examples, the component carrier which can provide allservices and to which a communication terminal must be connected at thevery least (the component carrier f1 in FIGS. 26 and 28) is sometimescalled a backward compatible component carrier. This is because it cansometimes support a communication terminal of Rel-8/9 and the like aswell (Non-patent document 2). Conversely, component carriers other thanthe above are sometimes called non-backward compatible componentcarriers.

While the downlink and uplink are not particularly distinguished fromeach other in the above description, the description basically centerson downlink operation. The downlink and uplink correspond one-to-onewith each other in LTE Rel-8.

FIG. 29 shows an “operation in LTE Rel-8.” That is, when a frequency 1used for the downlink and a frequency 4 used for the uplink are pairedwith each other and a communication terminal uses the frequency 1 toreceive, it uses the frequency 4 to transmit. Similarly, frequencies 2and 5, as well as frequencies 3 and 6, are paired with each other. Theprocess in FIG. 27 is thus shown only for the downlink for the purposeof simplification though different component carriers are actually usedfor reception and for transmission.

FIG. 30 shows an example of possible carrier aggregation in which thedownlink and uplink are asymmetric. There may also be such an asymmetricoperation in future extensions. However, the invention can be applied toeither case where the uplink and downlink are symmetric or asymmetric.The description below will center on downlink component carriers.

PRIOR ART DOCUMENTS Non-Patent Documents

-   Non-patent document 1: 3GPP TR 21.905 V8.8.0-   Non-patent document 2: R2-092866, “Synchronization channel and    system information for carrier aggregation”-   Non-patent document 3: 3GPP TS 36.331 V8.5.0-   Non-patent document 4: 3GPP TS 36.321 V8.5.0-   Non-patent document 5: 3GPP TS 36.101 V8.5.1

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As described above, whether to perform a cell search and qualitymeasurement or not is determined by using a search threshold value and,when carrier aggregation is performed, all component carriers comprisingthe carrier aggregation are regarded as serving cells. A plurality ofserving cells thus exist, and therefore it is required to determine howto make a comparison to a search threshold value.

A purpose of the invention is to provide a communication terminal and abase station which are capable of determining a search threshold valuefor carrier aggregation to determine the timing of starting a cellsearch.

Means for Solving the Problems

A communication terminal of the invention is capable of simultaneouslycommunicating through a plurality of carriers by carrier aggregation,and comprises: a quality measurement unit for measuring the receptionquality of a radio wave transmitted through a plurality of carriers froma base station of a connected cell to obtain a measured value; a primarycarrier storage unit storing information specifying a primary carrierchosen from the plurality of carriers; a comparator for comparing ameasured value of the primary carrier measured by the qualitymeasurement unit to a threshold value; and a cell search unit forsearching for another cell when the measured value of the primarycarrier is less than or equal to the threshold value.

Advantages of the Invention

The invention involves determining the timing of performing a cellsearch based on the reception quality of a primary carrier, and therebyallows even a communication terminal supporting carrier aggregation toappropriately start a cell search.

There are other aspects of the invention as described below. Thisdisclosure of the invention therefore intends to provide part of theaspects of the invention and does not intend to limit the scope of theinvention described and claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the timing for a communication terminal according to afirst embodiment to perform a cell search;

FIG. 2 shows a configuration of the communication terminal of the firstembodiment;

FIG. 3 shows an operation of the communication terminal of the firstembodiment;

FIG. 4 shows a scenario example of carrier aggregation;

FIG. 5 shows another scenario example of carrier aggregation;

FIG. 6 shows a process of specifying a primary component carrier;

FIG. 7 shows another operation of a communication terminal to determinea primary component carrier;

FIG. 8 shows a configuration of a base station;

FIG. 9 illustrates frequency bands;

FIG. 10 shows the timing for a communication terminal according to asecond embodiment to perform a cell search;

FIG. 11 shows a configuration of the communication terminal of thesecond embodiment;

FIG. 12 shows a process of specifying a primary component carrier;

FIG. 13 shows an operation of the communication terminal of the secondembodiment;

FIG. 14 shows an example of carrier-aggregated frequency bands andfrequency bands to be measured;

FIG. 15 shows the timing for a communication terminal according to athird embodiment to perform a cell search;

FIG. 16 shows a configuration of the communication terminal of the thirdembodiment;

FIG. 17 shows the timing of gaps set by the communication terminal ofthe third embodiment;

FIG. 18 shows the timing of gaps set by the communication terminal ofthe third embodiment;

FIG. 19 shows an operation of the communication terminal of the thirdembodiment;

FIG. 20 shows the timing for a communication terminal according to afourth embodiment to perform a cell search;

FIG. 21 shows the timing for the communication terminal according to thefourth embodiment to perform a cell search;

FIG. 22 shows a configuration of the communication terminal of thefourth embodiment;

FIG. 23 shows an operation of the communication terminal of the fourthembodiment;

FIG. 24 illustrates a search threshold value;

FIG. 25 is a conceptual diagram illustrating carrier aggregation;

FIG. 26 shows one scenario for further enhancing the efficiency ofcarrier aggregation;

FIG. 27 shows a process of adding component carriers;

FIG. 28 shows another scenario for further enhancing the efficiency ofcarrier aggregation;

FIG. 29 shows an operation in LTE Rel-8;

FIG. 30 shows an example of possible carrier aggregation in which thedownlink and uplink are asymmetric;

FIG. 31 shows a scenario example of carrier aggregation;

FIG. 32 shows the timing for a communication terminal according to afifth embodiment to perform a cell search;

FIG. 33 shows a configuration of the communication terminal according tothe fifth embodiment; and

FIG. 34 shows an operation of the communication terminal according tothe fifth embodiment.

MODE OF EMBODYING THE INVENTION

The following is a detailed description of the invention. Theembodiments described below are only examples of the invention, and theinvention can be varied in various aspects. Therefore, the specificconfigurations and functions disclosed below do not limit the claims.

Now, a communication terminal and base station of embodiments of theinvention will be described with reference to the drawings.

(First Embodiment)

FIG. 1 shows the timing for a communication terminal 1 according to afirst embodiment to perform a cell search. A primary component carrieris already determined among component carriers performing carrieraggregation. A component carrier f2 is the primary component carrier inthe example shown in FIG. 1. The primary component carrier may bespecified by a base station 30 or may be determined by the communicationterminal 1 based on some kind of rule. The method of determination willbe described later.

The communication terminal 1 compares a measured value of the receptionquality of the primary component carrier to a search threshold valueand, if the measured value of the primary component carrier is more thanor equal to the search threshold value, determines a cell search to beunnecessary even if any measured value of other component carriers arebelow the search threshold value. Conversely, if the measured value ofthe primary component carrier is below the search threshold value, thecommunication terminal 1 determines a cell search to be required.

[Communication Terminal]

FIG. 2 shows a configuration of the communication terminal 1 of thefirst embodiment. The communication terminal 1 has a receiver 11, ameasurement configuration unit 12, a primary carrier determination unit13, a primary carrier storage unit 14, a quality measurement unit 15, acomparator 16, a cell search unit 17, a measurement result judging unit18, and a transmitter 19.

The receiver 11 receives a signal transmitted from the base station 30.From among the received information, the receiver 11 sends informationon measurement (a measurement configuration) to the measurementconfiguration unit 12 and sends information for determining a primaryfrequency to the primary carrier determination unit 13. The receiver 11also sends a signal for measurement transmitted from the base station 30separately to the quality measurement unit 15 and cell search unit 17.

The measurement configuration unit 12 processes the information onmeasurement sent from the receiver 11 and configures therewith thequality measurement unit 15, the comparator 16, the cell search unit 17,and the measurement result judging unit 18. Specific examples of theinformation to be processed here include measConfig of IE (InformationElements) in an RRC Connection Reconfiguration message provided inNon-patent document 3. MeasConfig includes a frequency/cell to bemeasured (which is called and hereinafter referred to as a “measurementobject”), information on how to report a measurement result to the basestation 30 (which is called and hereinafter referred to as a “reportingconfiguration”), information on how to measure (which is called a“quantity configuration” and is hereinafter referred to as a“measurement configuration”), a search threshold value, and the like.

The measurement configuration unit 12 also notifies the qualitymeasurement unit 15 and cell search unit 17 of a measurement object, ameasurement configuration, and the like, notifies the measurement resultjudging unit 18 of a reporting configuration and the like, and notifiesthe comparator 16 of the search threshold value mentioned above.

The primary carrier determination unit 13 determines which one of aplurality of carrier-aggregated component carriers is to be used as aprimary component carrier for a comparison to the search thresholdvalue. The method of determining a primary component carrier will bedescribed later in detail. The primary carrier determination unit 13stores the result of primary component carrier determination in theprimary carrier storage unit 14.

The quality measurement unit 15 performs measurement on a currentlyconnected component carrier as configured by the measurementconfiguration unit 12. The quality measurement unit 15 sends themeasurement result to the comparator 16 and the measurement resultjudging unit 18.

The comparator 16 reads the information on a primary carrier from theprimary carrier storage unit 14 to specify a primary component carrier.The comparator 16 then compares the quality measurement result for theprimary component carrier to the search threshold value passed from themeasurement configuration unit 12 and determines whether or not to starta cell search. The comparator 16 notifies the cell search unit 17 of thejudgment result.

When a cell search is determined to be performed by the comparisonresult received from the comparator 16, the cell search unit 17 performsa cell search according to the details configured by the measurementconfiguration unit 12 and performs quality measurement for a detectedcell. The cell search unit 17 sends the measurement result to themeasurement result judging unit 18.

The measurement result judging unit 18 compares the measurement resultsreceived from the quality measurement unit 15 and cell search unit 17and, based on the measurement configuration configured by themeasurement configuration unit 12, determines whether a report to thebase station 30 is to be made or not. If the reporting is determined tobe made, the measurement result judging unit 18 creates a MeasurementReport message and sends it to the transmitter 19. The transmitter 19transmits the Measurement Report message passed from the measurementresult judging unit 18 to the base station 30.

FIG. 3 shows a flowchart showing an operation of the communicationterminal 1 of the embodiment. The measurement configuration unit 12 ofthe communication terminal 1 receives a measurement configurationtransmitted from the base station 30, and passes configuration values inthe received measurement configuration information on to the qualitymeasurement unit 15, comparator 16, cell search unit 17, and measurementresult judging unit 18 to configure them with the configuration values(S10).

The primary carrier determination unit 13 of the communication terminal1 then determines a primary component carrier to be compared to a searchthreshold value (S12). The primary carrier determination unit 13receives information on the primary component carrier from the basestation 30, and determines the received component carrier to be aprimary. The primary carrier determination unit 13 stores information onthe determined primary carrier in the primary carrier storage unit 14.

The comparator 16 of the communication terminal 1 then judges whether ameasured value of the reception quality of the primary component carrieris more than or equal to the search threshold value or not (S14). Thecomparator 16 receives information on the measured value of thereception quality of the primary component carrier from the qualitymeasurement unit 15. If the measured value of the primary componentcarrier is more than or equal to the search threshold value (Yes atS14), the communication terminal 1 does not perform a cell search butmonitors the measured value of the primary component carrier until itfalls below the search threshold value.

If the measured value of the primary component carrier is below thesearch threshold value (No at S14), the communication terminal 1 startsa cell search (S16). If the communication terminal 1 detects aneighboring cell, it performs quality measurement for the detected cell.

How to determine a primary component carrier will be described below. Asdescribed above with the help of FIGS. 26 and 28, there are a backwardcompatible component carrier, which can alone provide services, andother component carriers in a carrier aggregation operation. Thecommunication terminal 1 needs to receive broadcast information and anL1 control channel, and therefore a good reception quality requires tobe always kept on a backward compatible carrier. For that reason, abackward compatible carrier may be set as a primary component carrier.

FIG. 4 shows a scenario example of carrier aggregation. In such a casewhere there is only one backward compatible component carrier (f3 inthis example) when the communication terminal 1 performs carrieraggregation, the component carrier is handled as a primary componentcarrier and is used for a comparison to the search threshold value.

FIG. 5 shows another scenario example of carrier aggregation. In thisexample, a plurality of backward compatible component carriers f1 and f3are assigned to the communication terminal 1. One of the two backwardcompatible component carriers is used as a primary component carrier inthis case.

FIG. 6 shows a process of specifying a primary component carrier. At thebeginning, the communication terminal 1 camps on a component carrier f1and is on standby (S20). When a trigger for changing from an idle stateto an active state is activated by the communication terminal 1receiving paging or making a phone call (S22), the communicationterminal 1 uses the component carrier f1 to transmit Random AccessPreamble to the base station 30 (S24). Receiving this, the base station30 transmits Random Access Response to the communication terminal 1(S26).

The communication terminal 1 then transmits an RRC Connection Requestmessage to the base station 30 (S28), which upon receiving thistransmits an RRC Connection Setup message to the communication terminal1 (S30). This causes the communication terminal 1 to change from an idlestate to an active state (S32) and connect to the component carrier f1(S34).

Subsequently, the communication terminal 1 transmits an RRC ConnectionComplete message to the base station 30 (S36), which transfers the RRCConnection Complete message to its core network (S38). Receiving this,the core network device 40 transmits to the base station 30 capabilityinformation including information on whether the communication terminal1 can perform carrier aggregation or not (S40). Receiving the capabilityinformation for the communication terminal 1, the base station 30determines to operate carrier aggregation (S42).

If the base station 30 determines to perform carrier aggregation, ittransmits to the communication terminal 1 a Security Mode Command (S44)and, subsequently, an RRC Connection Reconfiguration message (S46). TheRRC Connection Reconfiguration message transmitted here includes aninstruction to add component carriers f2 and f3 and an instruction tospecify a primary component carrier.

Receiving the RRC Connection Reconfiguration message, the communicationterminal 1 adds the component carriers f2 and f3 (S48 and S50), andtransmits Security Mode Complete to the base station 30 (S52).Subsequently, the communication terminal 1 transmits an RRC ConnectionReconfiguration Complete message (S54).

While an example where a primary component carrier is explicitlyspecified is shown in FIG. 6, a primary component carrier may also bedetermined from other information. For example, the communicationterminal 1 is at the beginning connected to the component carrier f1 inthe example shown in FIG. 6 and, in such a case, the component carrierf1 may be handled as a primary unless otherwise specified.

FIG. 7 shows a flowchart of the above-described operation of thecommunication terminal 1 determining a primary component carrier. First,the communication terminal 1 judges whether there is carrier aggregationor not (S60). If there is no carrier aggregation (No at S60), there isonly one component carrier, and therefore the communication terminal 1sets the carrier that it uses for connection as a primary (S62).

If there is carrier aggregation (Yes at S60), the communication terminal1 judges whether there is explicit assignment of a primary componentcarrier or not (S64). If there is explicit assignment (Yes at S64), thecommunication terminal 1 sets the explicitly assigned component carrieras a primary (S66).

If there is no explicit assignment (No at S64), the communicationterminal 1 judges whether the carrier-aggregated carriers include morethan one backward compatible carriers or not (S68). If there is only onebackward compatible carrier (No at S68), the communication terminal 1sets the backward compatible carrier as a primary carrier (S70). Ifthere is a plurality of backward compatible carriers (Yes at S68), thecommunication terminal 1 sets the component carrier that it has beenconnected to from the beginning as a primary (S72).

[Base Station]

FIG. 8 shows a configuration of the base station 30 of the embodiment.The base station 30 of the embodiment has a function to specify aprimary component carrier for the communication terminal 1. The basestation 30 has a terminal information manager 31, an aggregationdetermination unit 32, a primary determination unit 33, a configurationdetermination unit 34, and a transmitter 35.

The terminal information manager 31 manages a channel configuration,capability, and the like of the communication terminal 1. The terminalinformation manager 31 sends these pieces of information to theaggregation determination unit 32.

The aggregation determination unit 32 determines whether thecommunication terminal 1 performs carrier aggregation or not, determineson which component carriers the aggregation is to be performed ifcarrier aggregation is to be performed, and sends the result to theprimary determination unit 33.

The primary determination unit 33 determines a primary componentcarrier, and sends information on the determined primary componentcarrier and the information received from the aggregation determinationunit 32 both to the configuration determination unit 34.

The configuration determination unit 34 creates a message forinstructing the communication terminal 1 which component carrier is tobe a primary, and sends the message to the transmitter 35. Note herethat the instruction on a primary component carrier will be omitted if,in the setting process of the primary component carrier of thecommunication terminal 1 shown in FIG. 7, the communication terminal 1selects a component carrier that is desired by the base station 30 to bea primary without any instruction from the base station 30. Thetransmitter 35 transmits the message created by the configurationdetermination unit 34 to the communication terminal 1. This is adescription of configurations and operations of the communicationterminal 1 and base station 30 of the first embodiment.

Since the communication terminal 1 of the embodiment determines thetiming of performing a cell search based on the reception quality of aprimary carrier, it can appropriately start a cell search even whencarrier aggregation is performed with a plurality of component carriers.

Since the base station 30 of the embodiment transmits informationindicating a primary component carrier to the communication terminal 1,the communication terminal 1 can use an appropriate component carrier tomake a judgment on a cell search.

While the above description is of how to determine a primary componentcarrier, a primary component carrier may be determined by methods otherthan the above.

For example, a measurement object in a measurement configuration is tobe used to set a frequency/cell to be measured. If only one componentcarrier is specified here as a frequency to be measured, the frequencycan be determined to be a primary component carrier. Conversely, theremay also be a configuration in which a component carrier that was aprimary is excluded from being a measurement object. In such a case, thecomponent carrier concerned may cease to be a primary and anotherbackward compatible component carrier may be set as a primary. In thiscase, if there are a plurality of other backward compatible componentcarriers, a primary component carrier is determined in accordance withan instruction from the base station 30.

Instead of receiving an instruction from the base station 30, thecommunication terminal 1 may control dynamically in such a manner thatit makes the highest-performing component carrier a primary or it makesthe lowest-performing component carrier a primary. The judgment of goodor bad performance here may be made by using a measurement result usedfor a Measurement Report message or by using as another choice aninstantaneous measurement result used for CQI reporting.

The component carrier most used for reception among carrier-aggregatedcomponent carriers may be set as a primary.

The handling as a primary component carrier may be applied not only tothe comparison to the search threshold value but also to the reportingof a Measurement Report message. Since a Measurement Report message willconstantly report the quality of a serving cell, a primary componentcarrier determined in the embodiment may be used as the componentcarrier to be reported then.

Discontinuous Reception (DRX), which is performed by the communicationterminal 1, may also be considered. During carrier aggregation, DRX isnot similarly performed on all component carriers but may be operatedfor each component carrier. Specifically, for example, DRX is performedonly on a component carrier that is not used a lot, and is not performedon a frequently-used component carrier. In such a case where the DRXoperation differs from one component carrier to another, there may be anoperation in which a component carrier that DRX is performed on is notset as a primary component carrier. Conversely, a component carrier setas a primary may start DRX on condition that all the other componentcarriers start DRX. As a result, even when a component carrier set as aprimary has not been used for a while, the component carrier set as aprimary will not start DRX as long as another component carrier has beenused. A component carrier set as a primary is presumed to have a good orstable quality for its communication terminal or as a system, and it isdesirable that a component carrier set as a primary be used as much aspossible and that DRX be performed thereon less frequently than on theother component carriers. The above-described operation allows acomponent carrier set as a primary to be kept in a state where it can beused at all times even if communication cannot be established with acertain communication terminal by using the component carrier set as aprimary when the component carrier set as a primary is under high load.

(Second Embodiment)

A communication terminal 2 of a second embodiment will be describedbelow. The communication terminal 2 of the second embodiment isdifferent from that of the first embodiment in that it controls theoperation for each frequency band.

FIG. 9 illustrates frequency bands. FIG. 9 shows carriers f1 to f5 ascandidates for component carriers. A plurality of carriers of thecarriers f1 to f5 are used to perform carrier aggregation. Note herethat the carriers f1 and f2 are included in a first frequency band (e.g.800 MHz band) and the carriers f3 to f5 are included in a secondfrequency band (e.g. 2 GHz band). In an operation like this, it isdesirable to control the operation for each frequency band since theremay be a backward compatible carrier in each frequency band andmeasurement results may differ considerably from one frequency band toanother. In the embodiment, a primary component carrier is selected foreach frequency band.

FIG. 10 shows the timing for the communication terminal 2 according tothe second embodiment to perform a cell search. This is an example wherecarrier aggregation is performed by using, as component carriers, thecarriers f1 to f3 of the carriers f1 to f5 shown in FIG. 9. Thecomponent carrier f1 of the component carriers f1 and f2 included in thefirst frequency band is a primary. Since the second frequency bandincludes only the component carrier f3, this is a primary.

As shown in FIG. 10, a cell search is not performed if the quality isabove the search threshold value for all the primary component carriers(the component carriers f1 and f3 in this example). A cell search isstarted for the second frequency band when the quality of the primary ofthe second frequency band, the component carrier f3, falls below thesearch threshold value. Specifically, the search process is performedonly for f3 if only f3 is in the measurement configuration for thesecond frequency band or, if there is any carrier other than f3 (e.g. f4shown in FIG. 9) in the configuration and if the communication terminal2 can perform the measurement for the other frequency (i.e. f4) whilekeeping the connection to the component carrier it is connected to, thesearch process is performed for f4 as well as for f3. Since the qualityof the component carrier f1 is above the search threshold value at thispoint in time, a cell search is not performed on the component carriersincluded in the first frequency band.

The process is the same (that is, a cell search is not performed) evenif the quality of the component carrier f2, which is not the primary ofthe first frequency band, falls below the search threshold value. A cellsearch is performed on all the component carriers in the configurationfor the communication terminal 2 including the first frequency band onlywhen the quality of the primary of the first frequency band, thecomponent carrier f1, falls below the search threshold value.

FIG. 11 shows a configuration of the communication terminal 2 of thesecond embodiment. The configuration of the communication terminal 2 ofthe second embodiment is basically the same as that of the firstembodiment, but is different therefrom in that it has a plurality ofcomparators 16. A description will be given below of the configurationof the communication terminal 2 of the second embodiment, centering ondifferences with the first embodiment.

The primary carrier determination unit 13 determines a primary componentcarrier for each frequency band. The primary carrier determination unit13 notifies the comparator 16 for each frequency band of information onthe determined primary component carrier.

The quality measurement unit 15 notifies the plurality of comparators 16of quality measurement results. The cell search unit 17 does not start acell search for all at once, but controls the start of a cell search foreach frequency band in accordance with instructions from the pluralityof comparators 16.

Each of the plurality of comparators 16 compares the primary componentcarrier of the corresponding frequency band to the search thresholdvalue. Upon receiving the reception quality of the primary componentcarrier of the corresponding frequency band from the quality measurementunit 15, each of the plurality of comparators 16 compares the receptionquality to the search threshold value and notifies the cell search unit17 of the comparison result.

FIG. 12 shows a process of specifying a primary component carrier in thesecond embodiment. The details of the process are basically the same asthose in the first embodiment shown in FIG. 6, but are differenttherefrom in that a primary component carrier is specified in RRCConnection Reconfiguration message (S46 a) for each frequency band.

FIG. 13 is a flowchart showing an operation of the communicationterminal 2. The communication terminal 2 receives information on ameasurement (a measurement configuration) transmitted from the basestation 30, and determines the measurement configuration by means of themeasurement configuration unit 12 (S80). The communication terminal 2then judges whether a plurality of frequency bands are used in carrieraggregation or not (S82). If a plurality of frequency bands are judgednot to be used (No at S82), the communication terminal 2 determines aprimary component carrier (S84). The method described in the firstembodiment can be used for this.

If a plurality of frequency bands are judged to be used (Yes at S82),the communication terminal 2 determines a primary component carrier foreach frequency band (S86).

The comparators 16 of the communication terminal 2 then judge whetherthe quality of the primary component carriers is more than or equal tothe search threshold value or not (S88). If the quality of the primarycomponent carriers is more than or equal to the search threshold value(Yes at S88), the communication terminal 2 does not perform a cellsearch but monitors the quality of the primary component carriers untilit falls below the search threshold value. If the quality of any primarycomponent carrier is not more than or equal to the search thresholdvalue (No at S88), the communication terminal 2 starts a cell search forthe frequency band including the component carrier that is below thesearch threshold value (S90).

The base station 30 of the second embodiment will be described below.The configuration of the base station 30 of the second embodiment isbasically the same as that of the first embodiment (see FIG. 8).However, the base station 30 of the second embodiment determines aprimary component carrier for each frequency band by means of theprimary determination unit 33 and transmits them from the transmitter35.

Groups divided according to frequency bands in the embodiment will bedescribed in detail here. These groups include a bundle of neighboringcomponent carriers. Specifically, component carriers can be groupeddepending on frequency bands according to (1) to (3) given below. Themethods shown below do not contradict each other, and can be combined inany way.

(1) As shown in Chapter 5.7.3 of Non-patent document 5, correspondingfrequencies are defined in LTE. In this case, frequencies bundledtogether as E-UTRA Operating Band may be regarded as a group of the samefrequency band.

(2) The base station 30 may determine groups of the same frequency bandsin accordance with the system operation. In this case, the base station30 notifies the communication terminal 2 of the range of frequency bandsof the same groups. This notification may be transmitted throughbroadcast information or through messages specific to communicationterminals.

(3) A rule may be made according to which only a certain frequency bandis regarded as the same group. Specifically, for example, with a 100 MHzband being defined in advance (e.g. 95-105 MHz), those included in thisband are regarded as being in the same frequency band and those notincluded in this band are regarded as being in another frequency band.

When carrier aggregation is performed by using different frequencybands, the communication terminal 2 of the embodiment can control a cellsearch in consideration of differences in reception quality caused bythe difference of frequency band. This can reduce power consumption ofcommunication terminals.

The embodiment has been shown with an example in which a common value isset as the search threshold value regardless of frequency band. However,search threshold values different from one group to another depending onfrequency band may be set.

The start of a cell search is in the embodiment controlled for eachfrequency band. However, a cell search may be performed on all componentcarriers if any of the primary component carriers falls below the searchthreshold value. Conversely, there may be an operation in which a cellsearch is not started until all the primary component carriers fallbelow the search threshold value.

(Third Embodiment)

A communication terminal 3 of a third embodiment will be describedbelow. The communication terminal 3 of the third embodiment, unlike thesecond embodiment, performs a cell search not only on carrier-aggregatedcomponent carriers but also on non-carrier-aggregated carriers when aprimary component carrier falls below the search threshold value. Theconfiguration of the base station 30 of the third embodiment is the sameas that of the second embodiment.

FIG. 14 shows an example of carrier-aggregated frequency bands andfrequency bands to be measured. As shown in FIG. 14, a carrieraggregation is configured with component carriers f1, f2, and f5, and ameasurement object is configured with the component carriers f1, f2, andf5, in addition to a component carrier f3.

In this case, since the component carriers f1, f2, and f5 are currentlyconnected component carriers, the communication terminal 3 can perform acell search and a quality measurement on the same component carriers f1,f2, and f5 for a neighboring cell at the same time on the componentcarriers f1, f2, and f5 for the connected cell. However, the componentcarrier f3 is not currently included in the carrier aggregation. Forexample, in a case where the communication terminal 3 can be connect toonly three component carriers at the same time, that is, where thenumber of component carriers with which the communication terminal 3 canperform carrier aggregation, the capability, is three, a cell search andquality measurement cannot be performed on the component carrier f3unless the communication with any one of the component carriers f1, f2,and f5 is interrupted.

Generally, this interruption process cannot be performed unless gapconfiguration is performed in which the base station 30 explicitlyprovides the timing of gaps for the communication terminal 3. Theembodiment allows a cell search to be performed on a component carrierthat is not in a band aggregation like the component carrier f3, withoutan instruction from the base station 30.

FIG. 15 shows the timing for the communication terminal 3 according tothe third embodiment to perform a cell search. As with the first andsecond embodiments described above, a cell search is not performed whenthe quality of all component carriers is above the search thresholdvalue.

Then when the quality of the component carrier f5 in the third frequencyband falls below the search threshold value, the communication terminal3 starts a cell search on the component carrier f5 as in the secondembodiment.

At the same time, the communication terminal 3 of the embodiment checksthe data reception mode of the component carrier f5. Specifically, thecommunication terminal 3 checks whether the component carrier f5 isconfigured for DRX (Discontinuous reception) or not and whether therehas been no data reception for a while or not. In such reception mode,it may not be indispensable to continuously maintain the connection onthe component carrier f5. Along with the cell search process on thecomponent carrier f5, the communication terminal 3 automatically setsgap periods for the component carrier f5, and performs a cell search onthe non-carrier-aggregated component carrier f3.

This allows a cell search to be performed on another component carrierwithout an instruction to set gaps from the base station 30 even undercircumstances where it is not used for carrier aggregation and a cellsearch cannot be performed on extra component carriers because of thelimit of the ability of the communication terminal 3 (the number ofcomponent carriers on which the communication terminal 3 can beconnected at the same time).

FIG. 16 shows a configuration of the communication terminal 3 of thethird embodiment. The configuration of the communication terminal 3 ofthe third embodiment is basically the same as that of the secondembodiment, but is different therefrom in that the communicationterminal 3 of the third embodiment has a reception mode detector 20. Adescription will be given below of the configuration of thecommunication terminal 3, centering on differences with the secondembodiment.

At the start of a cell search, the comparators 16 instruct the receptionmode detector 20 about a component carrier on which the cell search isdetermined to be performed, and instruct the reception mode detector 20to check the reception mode of the component carrier concerned.

Based on the instructions from the comparators 16, the reception modedetector 20 checks the reception mode of the specified componentcarrier. The reception mode is checked here about the presence orabsence of previously described DRX, about the presence or absence ofdata reception, and so on. If the judgment result indicates that DRX isperformed or that data has not been received for a certain period oftime, the reception mode detector 20 determines that the communicationterminal 3 can set gap periods for the component carrier concerned, andnotifies the cell search unit 17 of the information.

Based on the notification from the reception mode detector 20, the cellsearch unit 17 automatically sets gap periods for the currentlycarrier-aggregated component carrier, and performs a cell search on anon-carrier-aggregated component carrier.

FIG. 17 shows the timing of gaps set by the communication terminal 3 ofthe embodiment. Time t1 is a time when data was received last on thecomponent carrier f5. Time t2 is a time when a cell search was startedon the component carrier f5. This is a time when the quality of thecomponent carrier f5 fell below the search threshold value in FIG. 15.

The communication terminal 3 checks at the time t2 the status of thiscomponent carrier. If elapsed time from the time t1 (the timer T in FIG.17) exceeds a certain period of time (the period X in the figure), gapperiods are set for the component carrier f5 at the time point, and acell search is performed on the component carrier f3.

The “certain period of time” is a threshold value for determiningwhether or not to create gaps. This certain period of time X may benotified of to the communication terminal 3 through broadcastinformation (a form like a notification such as an IE “T300” transmittedin SIB2 described in Non-patent document 3), or may be notified ofindividually to the communication terminal 3 (a notification like an IE“drx-InactivityTimer” etc. included in an RRC Connection Reconfigurationmessage described in Non-patent document 3), or may be a fixed value.

In an example shown in FIG. 18, the timer T has not yet reached theperiod X at the time t2. When the timer T reaches the period X (timet3), gaps are set and a cell search is started on the component carrierf3.

The length and timing of the gap periods may be set freely by thecommunication terminal 3, or may be set by the communication terminal 3based on information specified by the base station 30. If the basestation 30 specifies them, it may specify the length of one gap, thecycle of creating gaps, and the timing of creating gaps (e.g. from whichsubframe in SFN (System Frame Number) mod=Y, etc.). This setting may benotified of through broadcast information, or may be notified ofindividually to the communication terminal 3.

In FIGS. 17 and 18 an operation is shown where whether gap periods canbe set or not is judged on whether data has been received on thecomponent carrier or not but, as described above, gap periods can be setif the DRX operation is performed. The DRX operation is provided inNon-patent document 4. In this case, gap periods are set so that a cellsearch is performed on the component carrier f3 at a time when thecommunication terminal 3 is not receiving data from the componentcarrier f5.

FIG. 19 shows an operation of the communication terminal 3 of the thirdembodiment. The communication terminal 3 receives information onmeasurement (a measurement configuration) transmitted from the basestation 30, and determines the measurement configuration by means of themeasurement configuration unit 12 (S100). The communication terminal 3then judges whether a plurality of frequency bands are used in carrieraggregation or not (S102).

If a plurality of frequency bands are judged not to be used (No atS102), the communication terminal 3 determines a primary componentcarrier (S104). The method described in the first embodiment can be usedfor this. If a plurality of frequency bands are judged to be used (Yesat S102), the communication terminal 3 determines a primary componentcarrier for each frequency band (S106).

The comparators 16 of the communication terminal 3 then judge whetherthe quality of the primary component carriers is more than or equal tothe search threshold value or not (S108). If the quality of the primarycomponent carriers is more than or equal to the search threshold value(Yes at S108), the communication terminal 3 does not perform a cellsearch but monitors the quality of the primary component carriers untilit falls below the search threshold value. If the quality of any primarycomponent carrier is not more than or equal to the search thresholdvalue (No at S108), the communication terminal 3 judges whether there isa carrier to be measured other than the carrier-aggregated componentcarriers or not (S110). If the result of this judgment indicates thatthere is no other carrier to be measured, the communication terminal 3starts a cell search for the frequency band including the componentcarrier that is below the search threshold value (S112).

If there is a carrier to be measured other than the carrier-aggregatedcomponent carriers, the communication terminal 3 judges whether acondition for setting gap periods for performing a cell search on thecarrier is met or not (S114). The condition for setting gap periods is acondition in which data has not been received for a period of time orDRX reception is performed on the component carrier that is below thesearch threshold value.

If the condition for setting gap periods is not met (No at S114), thecommunication terminal 3 starts a cell search for the frequency bandincluding the component carrier that is below the search threshold value(S112). If the condition for setting gap periods is met (Yes at S114),the communication terminal 3 starts a cell search on a carrier to bemeasured that is not currently included in the carrier aggregation, aswell as a cell search on the frequency band that is below the searchthreshold value (S116). This is a description of a configuration and anoperation of the communication terminal 3 of the third embodiment.

Thus, if data has not been received for a period of time on acarrier-aggregated component carrier, a cell search can be performed, bysetting gap periods for the component carrier, on another carrier thatis not in the carrier aggregation with limited influence on thereception process in the current connection. The case where data has notbeen received for a period of time includes a case where DRX receptionis performed and therefore data has not been received during thereception intervals.

While in the above-described embodiment a cell search is performed if acondition for setting gap periods (DRX mentioned above etc.) is met,whether or not to measure for a component carrier may be controlled byassigning priorities to the component carriers, in addition to the abovecondition.

In the example shown in FIG. 15 for example, suppose that the priorityof the component carrier f3 is set high and the priority of thecomponent carrier f5 is set low. In this case, since the priority of thecomponent carrier f3 is higher than that of the component carrier f5, acell search is performed on the component carrier f3 as described in theembodiment. Conversely, it is possible not to perform a cell search onthe component carrier f3 if the priority of the component carrier f3 isset low and the priority of the component carrier f5 is set high.

The priority of each component carrier here is a concept close to thefrequency priority introduced in 3GPP Rel-8. The frequency priorityintroduced in 3GPP Rel-8, however, is the priority for the communicationterminal 3 in an idle state or the like to select a frequency when itperforms mobility control without any instruction from the network and,on the other hand, the priority in this example is different in that itis used for determining for which frequency a cell search ispreferentially performed when the communication terminal 3 performsmobility control with an instruction received from the network. For thisreason, this priority of each component carrier needs to be used alsofor the communication terminal 3 in an active state. The notificationmay be made to the communication terminal 3 by broadcast information, orby an individual message. The communication terminal 3 stores thepriority information notified of in a storage unit (a carrier prioritystorage unit) in advance so that it can read and use the information asrequired.

While in the embodiment a case has been shown in which a cell search isperformed on a component carrier in the same system, i.e. LTE, withoutgap periods specified by the base station, the measurement may be madefor another system such as UMTS, GSM, CDMA 2000, WiMAX, or the like.

While the embodiment has been described based on the second embodimentin which the search operation is determined for each frequency band, itmay be applied to the first embodiment which does not have the conceptof the search operation for each frequency band.

(Fourth Embodiment)

A communication terminal 4 of a fourth embodiment will be describedbelow. The configuration of the communication terminal 4 of the fourthembodiment is basically the same as that of the first embodiment, but isdifferent therefrom in that it uses a plurality of search thresholdvalues. The base station 30 of the fourth embodiment is the same as thatof the first embodiment (see FIG. 8).

FIGS. 20 and 21 show the timing for the communication terminal 4according to the fourth embodiment to perform a cell search. A primarysearch threshold value is used for a primary component carrier, and asecondary search threshold value is used for component carriers otherthan a primary.

This allows a cell search to be started when a primary component carrierfalls below the primary search threshold value as shown in FIG. 20 orwhen all component carriers except for the primary fall below thesecondary search threshold value as shown in FIG. 21.

FIG. 22 shows a configuration of the communication terminal 4 of thefourth embodiment. The configuration of the communication terminal 4 ofthe fourth embodiment is basically the same as that of the firstembodiment (see FIG. 2), but is different therefrom in that it has aprimary comparator 16 a and a secondary comparator 16 b. A descriptionwill be given below, centering on differences with the first embodiment.

The primary carrier determination unit 13 notifies the primarycomparator 16 a of a primary component carrier, and notifies thesecondary comparator 16 b of the other component carriers.

The primary comparator 16 a judges whether the primary component carrieris below the primary search threshold value or not, and notifies thecell search unit 17 of the judgment result. The secondary comparator 16b judges whether all the component carriers other than the primary arebelow the secondary search threshold value, and notifies the cell searchunit 17 of the judgment result.

The cell search unit 17 starts a cell search if it is notified to starta cell search by either the primary comparator 16 a or the secondarycomparator 16 b.

FIG. 23 is a flowchart showing an operation of the communicationterminal 4. The communication terminal 4 receives information onmeasurement (a measurement configuration) transmitted from the basestation 30, and determines the measurement configuration by means of themeasurement configuration unit 12 (S120). The communication terminal 4then determines a primary component carrier (S122). The method describedin the first embodiment can be used for this.

The comparator 16 a of the communication terminal 4 then judges whetherthe quality of the primary component carrier is less than or equal tothe primary search threshold value or not (S124). If the quality of theprimary component carrier is less than or equal to the primary searchthreshold value (Yes at S124), the communication terminal 4 starts acell search (S128).

If the quality of the primary component carrier is not less than orequal to the primary search threshold value (No at S124), the secondarycomparator 16 b of the communication terminal 4 judges whether all thecomponent carriers other than the primary are less than or equal to thesecondary search threshold value or not (S126).

If the quality of the component carriers other than the primary is lessthan or equal to the secondary search threshold value (Yes at S126), thecommunication terminal 4 starts a cell search (S128). If the quality ofthe component carriers other than the primary is not less than or equalto the secondary search threshold value (No at S126), the communicationterminal 4 does not perform a cell search, but returns to the process ofjudging the measured value of the primary component carrier (S124). Thisis a description of a configuration and an operation of thecommunication terminal 4 of the fourth embodiment.

Since the communication terminal 4 of the embodiment uses the secondarysearch threshold value, in addition to the primary search thresholdvalue, to judge the measured values of the component carriers other thanthe primary, it can detect another cell in less time when the quality ofall the component carriers other than the primary decreases.

While in the embodiment the condition is that all the component carriersother than the primary fall below the secondary search threshold value,it may be that one or a certain number of the component carriers otherthan the primary fall below the secondary search threshold value.

While in the invention a cell search is started if the quality of theprimary component carrier falls below the primary search threshold valueor if the quality of all the component carriers other than the primaryfalls below the secondary search threshold value, a cell search may bestarted if both conditions are met.

When a DRX operation is performed in which a communication terminal doesnot receive for a long while, the comparison operation may be combinedwith DRX and be changed to an operation in which the comparison is madeonly between the reception quality of a primary component carrier andthe primary search threshold value. This is because it is desired toreduce the number of performing the search process since a reduction inpower consumption is greatly required when DRX is performed. Anotherreason is that a failure in handover or the like does not have muchinfluence since there is no transmission and reception of data during aDRX operation in which a communication terminal does not receive for along while. As a method that produces an effect similar to this, theremay be an operation in which the secondary search threshold value islowered when DRX is performed.

(Fifth Embodiment)

A communication terminal 5 of a fifth embodiment will be describedbelow. The configuration of the communication terminal 5 of the fifthembodiment is basically the same as that of the first embodiment, but isdifferent therefrom in that it uses, for each component carrier, anindex that takes into account as the reception quality not only thereception strength but also interference.

The “S-measure,” defined in the previously described Non-patent document3, is provided by using RSRP (Reference Signal Received Power). ThisRSRP indicates the reception strength, which means that the need for aneighboring cell search process is determined by using the receptionstrength as the reception quality.

A possible weakness of this case in which the need for a neighboringcell search process is determined by using the reception strength isthat even if the quality of a serving cell is sufficient, there is aninterfering cell and the cell cannot be detected. This problem is shownin FIG. 31. Suppose here that there are component carriers f1 and f2,that the component carrier f1 is set as a primary, and that a femto basestation is installed on the component carrier f2. In the firstembodiment, the need for a search and quality measurement for aneighboring cell is judged by using as the reception quality thereception strength of the component carrier f1 set as the primary. Inthis case, a communication terminal may exist near the femto basestation on the component carrier f2 even when the quality of thecomponent carrier f1 is sufficient. The communication terminal does notperform a search and quality measurement for a neighboring cell,therefore cannot detect the presence of the femto base station, and maysuffer interference from the femto base station or may causeinterference to a communication terminal connected to the femto basestation. This problem may not be solved even by using as a searchthreshold value the reception quality based on the reception strength ofthe component carrier f2. This is because there may be interference fromthe femto base station even if the reception strength of a serving cellon the component carrier f2 is sufficiently high. A solution to this maybe the use of interference-considered reception quality as shown in FIG.32. RSRQ (Reference Signal Received Quality) described in Non-patentdocument 3 is the reception quality that takes into account not only thereception strength but also interference. Thus, an operation of making acomparison by using S-measure_RSRQ for each component carrier is definedin addition to the operation described in the first embodiment, in whicha comparison is made by using S-measure_RSRP based on RSRP for a primarycomponent carrier. That is, referring to the example in FIG. 32, theprimary component carrier frequency 1 is compared to the S-measure_RSRPand, when the quality of the component carrier f1 becomes worse than theS-measure_RSRP, all the configured search and measurement processes arestarted as in the first embodiment. In addition, the quality of thecomponent carriers f1 and f2 is compared to the S-measure_RSRQ and, whenthe quality of the component carrier f2 becomes worse than theS-measure_RSRQ, the configured search and measurement for a neighboringcell are started on the component carrier f2. This allows thecommunication terminal 5 to reliably detect a femto base station whenthere is one as shown in FIG. 31.

While a case is shown in FIG. 32 where the RSRQ of the component carrierf2 becomes worse first, there may be a case where the RSRQ of thecomponent carrier f1 becomes worse first to be worse than theS-measure_RSRQ. In such a case, it is also possible that a search andmeasurement for a neighboring cell is started on the component carrierf1 and not on the component carrier f2. Conversely, all the configuredneighboring cell search and measurement processes can also be performedif the quality of the primary component carrier becomes worse thaneither the S-measure_RSRP or the S-measure_RSRQ.

This S-measure_RSRQ is required only when there is a femto base stationor the like on the network side and its interference to a communicationterminal cannot be estimated based on a primary component carrier. Forthis reason, there may be an operation that does not use S-measure_RSRQ.The operation in that case may be according to the first embodiment notwith S-measure_RSRQ being transmitted but only with S-measure_RSRP. In acase where component carriers to install femto base stations on arelimited, it is also conceivable to compare only such component carriersto S-measure_RSRQ. One way to achieve this may be that a base stationnotifies a communication terminal of component carriers to be comparedto S-measure_RSRQ.

FIGS. 33 and 34 show a block diagram and a flowchart, respectively, ofthe communication terminal 5 for achieving the above operation.Differences with the above-described embodiments will be described belowwith reference to FIG. 33 first.

An RSRQ comparator 23 compares a quality measurement result for eachcomponent carrier to a search threshold value for RSRQ passed from themeasurement configuration unit 12, and determines whether to start acell search or not for each component carrier. The RSRQ comparator 23notifies the cell search unit 22 of the determination result. If thisoperation is to be used only for certain component carriers here, themeasurement configuration unit 12 specifies the certain componentcarriers.

The cell search unit 22 uses both comparison results received from thecomparator 16 and received from the RSRQ comparator 23 to determinewhether or not to perform a cell search. If the cell search unit 22determines to perform a cell search, it does so according to the detailsconfigured by the measurement configuration unit 12 and performs qualitymeasurement for a detected cell. The cell search unit 22 sends themeasurement result to the measurement result judging unit 18. Anoperation of the cell search unit 22 for determining whether or not toperform a cell search is shown in FIG. 34.

FIG. 34 shows an operation of the communication terminal 5. Adescription will be given below of an operation of the communicationterminal 5 for determining whether or not to perform a cell search,centering on differences with the above-described embodiments. Themeasurement configuration unit 12 of the communication terminal 5receives a measurement configuration transmitted from the base station30 and sets the configuration values (S10), and the primary carrierdetermination unit 13 determines a primary component carrier to becompared to a search threshold value (S12); the operation is so far thesame as that in the first embodiment. The communication terminal 5 thenjudges whether a measured value of the reception quality of the primarycomponent carrier is more than or equal to the search threshold value ornot (S13). This operation is almost the same as step S14 shown in FIG.3, but is different therefrom in that the operation goes on to step S15if the judgment result is “Yes.” In step S15, the communication terminal5 checks whether there is any component carrier whose quality is lessthan or equal to the search threshold value or not. Not RSRP, which isused in step S13, but RSRQ is used here for the comparison to the searchthreshold value. If there is a component carrier whose quality is lessthan or equal to the search threshold value in this step, a neighboringcell search and neighboring cell measurement are performed on thecomponent carrier concerned (S17).

The embodiment allows a terminal to detect an interfering cell even incircumstances such as those shown in FIG. 31, and to perform a process,for example, to stop the use of the component carrier f2. This canrealize an efficient carrier aggregation.

The concept of frequency band may be introduced into the embodiment.Specifically, in a case where there are component carriers f2 and f3 inthe same frequency band (e.g. 800 MHz band) and the two componentcarriers are used, if the quality (RSRQ in this case) of either one ofthe component carriers falls below a search threshold value(S-measure_RSRQ), a cell search may be performed on both componentcarriers f2 and f3 in the same frequency band. In addition, if there isanother component carrier f4 that is not currently used in the samefrequency band but is in the measurement configuration, the measurementfor the component carrier f4 may also be started.

While there have been described what are at present considered to bepreferred embodiments of the invention, various modifications andvariations may be made thereto, and it is intended that appended claimscover all such modifications and variations as fall within the truespirit and scope of the invention.

Industrial Applicability

The invention has an advantage of allowing even a communication terminalsupporting carrier aggregation to appropriately start a cell search, andis useful as a communication terminal, a base station, and the like thatsupport carrier aggregation.

DESCRIPTION OF THE SYMBOLS

-   1-4: Communication terminal-   11: Receiver-   12: Measurement configuration unit-   13: Primary carrier determination unit-   14: Primary carrier storage unit-   15: Quality measurement unit-   16: Comparator-   16 a: Primary comparator-   16 b: Secondary comparator-   17: Cell search unit-   18: Measurement result judging unit-   19: Transmitter-   20: Reception mode detector-   30: Base station-   31: Terminal information manager-   32: Aggregation determination unit-   33: Primary determination unit-   34: Configuration determination unit-   35: Transmitter

The invention claimed is:
 1. A communication terminal, comprising:communication circuitry which, in operation, communicates with a firstbase station through a plurality of carriers, including at least aprimary carrier and a secondary carrier, by carrier aggregation; andcontrol circuitry which, in operation: measures reception quality ofradio signals transmitted through the plurality of carriers from thefirst base station to obtain measured values; compares the measuredvalue of the primary carrier to a first threshold value; and comparesthe measured value of the secondary carrier to a second threshold value,wherein the control circuitry: when discontinuous reception is performedby the communication terminal, wherein periods during which thediscontinuous reception is performed by the communication terminalinclude no data transmission and reception intervals of determinedlength: when the measured value of the primary carrier is lower than thefirst threshold value, performs a cell search for a first correspondingcarrier having a same frequency as the primary carrier, of a firstneighboring cell, and for a second corresponding carrier, having a samefrequency as the secondary carrier, of a second neighboring cell; andwhen the measured value of the primary carrier is equal to or higherthan the first threshold value, does not perform the cell search; andwhen discontinuous reception is not performed by the communicationterminal, determines whether to perform the cell search based on themeasured value of the primary carrier and the measured value of thesecondary carrier.
 2. The communication terminal according to claim 1,wherein the communication terminal is connected to a serving primarycell through the primary carrier and is connected to a serving secondarycell through the secondary carrier, wherein the serving primary andsecondary cells are supported by the first base station.
 3. Thecommunication terminal according to claim 2 wherein the firstneighboring cell is different from the serving primary cell and thesecond neighboring cell is different from the serving secondary cell,wherein the first and second neighboring cells are target cells for thecommunication terminal to connect to through the first and secondcorresponding carriers and are supported by a second base stationdifferent from the first base station, and the control circuitry, inoperation, measures the reception quality of radio signals transmittedthrough the first and second corresponding carriers of the first andsecond neighboring cells.
 4. The communication terminal according toclaim 1, wherein the primary carrier is a backward compatible componentcarrier and the secondary carrier is a non-backward compatible componentcarrier.
 5. The communication terminal according to claim 1, wherein theprimary carrier is set by the first base station.
 6. The communicationterminal according to claim 1, wherein the primary carrier is set by thecommunication terminal.
 7. A method performed by a communicationterminal, the method comprising: controlling communication with a firstbase station through a plurality of carriers including at least aprimary carrier and a secondary carrier by carrier aggregation,measuring the reception quality of radio signals transmitted through theplurality of carriers from the first base station to obtain measuredvalues; comparing the measured value of the primary carrier to a firstthreshold value; comparing the measured value of the secondary carrierto a second threshold value; when discontinuous reception is performedby the communication terminal, wherein periods during which thediscontinuous reception is performed by the communication terminalinclude no data transmission and reception intervals of determinedlength: when the measured value of the primary carrier is lower than thefirst threshold value, performing a cell search for a firstcorresponding carrier, having a same frequency as the primary carrier,of a first neighboring cell, and for a second corresponding carrier,having a same frequency as the secondary carrier, of a secondneighboring cell; when the measured value of the primary carrier isequal to or higher than the first threshold value, not performing thecell search; and when the discontinuous reception is not performed bythe communication terminal, determining whether to perform the cellsearch based on the measured value of the primary carrier and themeasured value of the secondary carrier.
 8. The method according toclaim 7, wherein the communication terminal is connected to a servingprimary cell through the primary carrier and is connected to a servingsecondary cell through the secondary carrier, wherein the servingprimary and secondary cells are supported by the first base station. 9.The method according to claim 8, wherein the first neighboring cell isdifferent from the serving primary cell and the second neighboring cellis different from the serving secondary cell, wherein the first andsecond neighboring cells are target cells for the communication terminalto connect to through the first and second corresponding carriers andare supported by a second base station different from the first basestation, and the method comprises measuring the reception quality ofradio signals transmitted through the first and second correspondingcarriers of the first and second neighboring cells.
 10. The methodaccording to claim 7, wherein the primary carrier is a backwardcompatible component carrier and the secondary carrier is a non-backwardcompatible component carrier.
 11. The method according to claim 7,comprising: receiving from the first base station information that setsthe primary carrier.
 12. The method according to claim 7, comprising:setting the primary carrier according to a defined rule.